1. Field of the Invention
This invention relates to a method of developing a latent image, and more particularly to a developing method which does not create density irregularity in a visualized image and which provides a sharp image free of stain in the non-image area thereof.
2. Description of the Prior Art
In the electrophotographic method or the electrostatic printing method or the like, the magnet brush developing method, the cascade developing method, the fur brush developing method or the powder cloud developing method is known as a method for developing an electrostatic image formed on an image bearing member such as photosensitive medium or master paper and particularly the former two methods have been widely used in practical apparatuses.
Any of these developing methods has comprised bringing developer into contact with the entire surface of the image bearing member, namely, indiscriminately bringing developer into contact with both of the image area on the image bearing member which has an electrostatic charge image and the non-image area on the image bearing member which has no electrostatic charge image, and could be called the indiscriminate contact system. In such indiscriminate contact system, adherence of the developer to the non-image area of the image bearing member has unavoidably occurred to create stain (so-called fog) in the background portion of the developed image which has diminished the dignity of the visualized image.
The developer used in this development is generally classified into a two-component developer consisting of a mixture of carrier such as powdered iron or glass beads and toner and a one-component developer containing no carrier.
In the case of the two-component developer, it is difficult to maintain the mixture ratio always constant and density of the developed image is varied by a variation in this mixture ratio. Also, deterioration of the carrier is unavoidable and this also affects the developing density. Accordingly, in the case of the two-component developer, it has been difficult to maintain the uniformity of the density. Fog could be somewhat but not completely alleviated by application of a bias to the developing device.
In the case of the one-component developer, it has been possible to maintain the density of the developed image constant, but the fog as mentioned above has generally been unavoidable. Particularly, magnetic developer typical as the one-component developer has been used for development by the magnet brush developing method. However, in such development, fog has occurred probably due to the frictional contact of the magnet brush and a sharp image has been difficult to obtain. That is, the magnet brush is usually formed on a sleeve roller having a magnet roller or a magnet disposed therewithin and is used for development by rotation of that roller and therefore, the ensuing magnet brush further frictional contacts the once developed image area to disturb the developer adhering to the marginal portion of the image. By this, the marginal portion of the image has been disturbed and the sharpness of the image has been lost.
On the other hand, as a method which causes no fog in the visualized image, there is known the developing method disclosed in U.S. Pat. No. 3,232,190. This comprises causing a supporting member supporting insulative developer particles thereon to be opposed to an image bearing member bearing an electorstatic image thereon with a clearance of several millimeters maintained therebetween, maintaining the developer so as not to contact the non-image area while, on the other hand, causing the developer particles to fly to the image area to thereby effect development, and is called the toner transfer development.
According to this toner transfer development, only the image area is selectively developed without bringing the developer into contact with the non-image area of the image bearing member and such method can be called the selective imparting developing system in contrast with the indiscriminate contact system which is common to the aforementioned developing methods.
As such system, there have also been proposed methods different from the developing method disclosed in the afore-mentioned U.S. Pat. No. 3,232,190. They are, for example, the methods disclosed in U.S. Patent Applications Ser. Nos. 938,101; 938,494; 58,434 and 58,435 proposed by the assignee of the present invention. These Applications disclose the methods which eliminate fog and which comprises opposing a developer supporting member to a latent image bearing member with a very minute clearance therebetween, forming on the supporting member a one-component developer layer thinner than the minute clearance, and causing the developer to be transferred by utilizing an electrostatic attraction created only in the image area of the latent image surface.